Pretty good - like the description says it doesn't understand LESS functionality so including a mixin call within a class for example doesn't highlight perfectly but far better than just using a plain text editor

I've install this extension on VS2012 Update 1 to try out for my .less files in project and found that all my .less files still open in plain text. So, how can I open .less file and VS give me some CSS tool like in the description said?

I found some extensions for more complete support for Less in VS and uninstalled "CSS Is Less".
But after uninstalling, the ".less" file extension is still mapped to the CSS language service.
After uninstalling, ".less" files still produce CSS warnings in the Error List window which is annoying.
How can I remove the ".less" mapping that was added by the extension manually?

You can try running "devenv /setup".
(For reference, extensions don't control their install/uninstall. I think there was an overzealous optimization in the extensibility system where it would cache registry information but only invalidate it when a new extension was installed. It's been awhile since I worked on VS, so I could be misremembering.)

I didn't see any note about what changed in version 1.1. But being the foolhardy sort, I just updated, and it seems to not work at all any more. I haven't tried reinstalling/downgrading/anything yet, but thought I would at least ask what is supposed to be different in this update before trying to figure out why it's no longer working...

As the subject says: Is it possible to extend the EditorFactory to the point, so that you can add additional rules for .less files (e.g. nested classes, @-variables, intellisense, ...)? Just opening .less files with the CSS-Editor is a big step in the right direction but those shown errors are hiding "real" errors in my classes. I tried working on my own Extension for this, but didn't come very far as I found no entry-point for such an extension.
Any suggestions? :)